It has conventionally been practiced to separate and recover volatile organic compounds from exhaust gas containing the volatile organic compounds such as toluene, xylene, methylene chloride, and the like from the standpoint of air pollution prevention and effective utilization of resources.
For example, the activated carbon adsorption method has been utilized to recover the volatile organic compound such as an organic solvent or the like contained in exhaust gas generated by the manufacturing process of plastic products, synthetic fibers, and semiconductors or various operations in magnetic tape industry. In the activated carbon adsorption method, two types of devices are known in the art. One is a fixed bed solvent recovery device which is provided with a plurality of adsorption towers packed with activated carbon so as to alternately repeat the adsorption and desorption. The other is a fluidized bed solvent recovery device which drops activated carbon from the top of a tower, performs adsorption of solvent or the like while flowing the activated carbon by the raw gas fed from the bottom of the tower, and then causes a separately provided desorber to desorb the adsorbed activated carbon obtained at the tower bottom. The adsorption steps by these devices are carried out under atmospheric pressure, and the desorption steps are often carried out at atmospheric pressure or under reduced pressure using water vapor or nitrogen gas.
Among these adsorption methods, the particular operation is described by taking a fixed bed adsorption device for performing atmospheric pressure desorption as an example with reference to FIG. 7. When an untreated gas containing a volatile organic compound is fed into an adsorption tower (1), the volatile organic compound in the untreated gas is adsorbed by the activated carbon layer and then the purified exhaust gas is released into the atmosphere. Upon completion of adsorption in the adsorption tower (1), an untreated gas intake valve (2) and a treated gas exhaust valve (3) are closed, water vapor is blown into the adsorption tower (1) through an adsorbent regeneration gas supply valve (5), and then the adsorbed volatile organic compounds are desorbed and discharged in gaseous form together with the water vapor. The gas mixture (desorbed gas) is introduced into a condenser (16) and is condensed by cooling.
If insoluble in each other, water in the condensate and the recovered volatile organic compound are separated into the volatile organic compound phase and the aqueous phase due to a difference in specific gravity at a separation tank (13). Then, the water is discharged as a wastewater (20) and the volatile organic compound is recovered in a volatile organic compound recovery tank (12). Note that separation into the upper phase and the lower phase in the separation tank (13) is determined by the difference in specific gravity between water and the volatile organic compound, and thus, the relationship of upper and lower phase is not fixed.
However, if water and the recovered volatile organic compound are partially or fully soluble in each other, it is necessary to conduct distilled separation using a distillation device in order to separate them and recover the solvent alone. If an untreated gas contains a plurality of types of volatile organic compounds or if, the recovered volatile organic compounds are partially or fully soluble in each other, it is also necessary to conduct distilled separation using a distillation device in order to separate them for individual recovery. Typical examples of the volatile organic compound which is soluble in water include ethyl acetate, methyl ethyl ketone, isopropyl alcohol, and the like. These volatile organic compounds have been widely used in printing industry, painting industry, and the like. Although most of them have conventionally been burned at the time of disposal for preventing air pollution, it is expected in the future that these volatile organic compounds are effectively used by recycling (material recycling) as solvents or by recycling (energy recycling) as fuels alternative to fossil fuel.